I have the need to take the occasional macro photo, and rather than repurchasing the Canon EF 100 f/2.8L IS Macro lens, I looked into just getting an extension tube. I was on the Canon Forums and saw a used Canon EF 25 II extension tube for a pretty decent price, so I picked it up.
I’ve been messing around with it on both my Canon 5D Mark III and Canon EOS M (with the EF-EOS M adapter) and it’s fine for what it cost me.
While it does autofocus, it hunts a lot and often doesn’t get a focus lock, so manual focus is pretty much the primary method when deploying the extension tub.
Also, the extension tube cuts down the original minimum focal distance on the lens used by quite a bit. For instance, the minimum focus distance of the Canon EF 40mm f/2.8 STM is just under 12″, but with the EF 25 II, the focal distance is about 3″.
Not to mention the use of an extension tube actually causes there to be a maximum focusing distance, and in most cases, the maximum focus distance is very close to the minimum focus distance. This means there is a very small focusing range when establishing the shot.
But that said, it’s a cheap way to get some usable macro product photos without having to buy an actual macro lens.
The above is an example photo shot with the setup pictured earlier: Canon EF 40mm f/2.8 STM stacked on the Canon EF 25 II extension tube attached the the Canon EOS M using the EF-EOS M adapter. The above photo is a minor crop.